1. Field of the Invention
Embodiments of the invention relate to the field of circuit protection devices. More particularly, the present invention relates to a low voltage circuit protection device having a reduced breakdown voltage by separating a breakdown diode from a PNPN structure.
2. Discussion of Related Art
Circuit protection devices form an electrical connection with a component or components in a circuit to be protected. Certain of these protection devices are used to protect circuits from overvoltage transients when a voltage spike occurs. One type of device used for overvoltage protection is a Zener diode which is designed to have a specific reverse breakdown voltage which is the conduction voltage when the diode is reverse biased. This is controlled by doping of the p-n junction allowing electrons to tunnel from the p-type material to the n-type material. Another type of overvoltage protection device is an avalanche diode which operates at avalanche breakdown when an overvoltage condition occurs. Although Zener and avalanche diodes provide a simple voltage transient protection device, the maximum current through the reverse junction is limited by power dissipation.
FIG. 1 illustrates an exemplary prior art protection device 10 using a breakdown diode coupled to a PNPN structure. In particular, an N-type slice 12 is disposed between p-type diffusion layers 13, 14 and an N+ cathode 15 is disposed on p-type diffusion layer 14. This device also includes a buried N-type diffusion layer 16 within N-type slice 12 and metal layer 18. When a voltage is applied to device 10, the buried diode section breaks down and current flows beneath the N+ cathode 15 as indicated by the breakdown current path at 17. Once the N+ cathode 15 becomes forward biased, the PNPN structure switches on to a low voltage state. Although this device structure works well for protection voltages above about 50V, it does not work as well for lower voltages since the heavy doping of the buried N region 16 within the PNP transistor reduces the transistor gain preventing the device from switching on easily.
FIG. 2 illustrates an exemplary prior art protection device 20 that can accommodate a breakdown voltage of about 8V. In this configuration, the breakdown diode is separated from the PNPN structure. In particular, an N-type slice 22 is disposed between p-type diffusion layers 23, 24 and N+ cathodes are disposed on p-type diffusion layer 24. A high concentration p-type diffusion 26 is disposed on a buried N-type diffusion 27 to provide junction breakdown. The breakdown current path is indicated at 28. Because the breakdown diode is separated from the PNPN structure, much higher doping concentrations may be used in the breakdown diode without compromising the switching characteristics of the PNPN structure. This allows the breakdown voltage of the device 10 to be reduced to about, for example, 8V.
However, for certain protection applications even lower breakdown voltages are required. For example, circuits used to protect certain data lines require voltages lower than 8V. As shown in FIG. 3, each of a plurality of low voltage protection devices 311 . . . 31N is disposed between a respective data line 301 . . . 30N and ground 35 to protect an integrated circuit (IC) 32. In addition, protection for LED lamps which are configured in series also require lower breakdown voltages. This is particularly important since if one of the LED's in the series fails creating an open circuit in the series, the remaining LED's will not receive current rendering them inoperative. This problem may be overcome as shown in FIG. 4 by utilizing a plurality of low breakdown voltage protection devices 401 . . . 40N across corresponding LED's 411 . . . 41N connected in series. In this manner, if a particular one of the LED's 411 . . . 41N fails the corresponding low voltage protection device 401 . . . 40N will allow current to flow to the remaining LED's in the series. For example, if LED 41N-1 fails, corresponding low voltage protection device 40N-1 allows current to flow to remaining LED 40N. In this configuration, the switching voltage of each of the protection devices 401 . . . 40N should be above the running voltage of the corresponding LED 411 . . . 41N. By way of example, a minimum breakdown voltage of about 5V and a maximum of about 7V allows an associated LED driver circuit to supply enough voltage to switch the respective protection device into conduction. Unfortunately, fabrication processes used to manufacture IC's with breakdown voltages in these ranges are complicated as well as costly. Thus, there is a need for a discrete low voltage protection device that is less costly to manufacture, but that still satisfies desired reliability requirements.